1. Field of the Invention
The present invention relates to an image processing method and a computer readable medium for displaying a series of images using volume data.
2. Description of the Related Art
The medical field is revolutionized by the advent of CT (computed tomography) and MRI (magnetic resonance imaging), which enabled direct observation of the internal structure of a human body with progress in image processing technique using a computer. As a result, medical diagnosis using a tomogram of a living body is widely practiced. Furthermore, in recent years, volume rendering is used in medical diagnosis as a technique for visualizing a three-dimensional internal structure of a human body which is too complicated to understand with only tomograms. Volume rendering is a technique by which an image of a three-dimensional structure is directly rendered from volume data, which are three-dimensional digital data of an object obtained by CT, for example.
As three-dimensional image processing using the volume data, ray casting, MIP (Maximum Intensity Projection), MinIP (Minimum Intensity Projection), MPR (Multi Planar Reconstruction), and CPR (Curved Planar Reconstruction) are generally used. Further, a 2D slice image, etc., is generally used as two-dimensional image processing using the volume data.
FIG. 11 is a schematic representation in the MIP processing, in a case where a three-dimensional object 100 is swung from side to side and displayed so as to make it easy to determine an anteroposterior positional relationship of the three-dimensional object 100 as an observation object. The MIP processing is a method of the three-dimensional volume rendering, and in the MIP processing, rendering is performed based on a maximum value of the volume data on a virtual ray.
Usually, in the MIP processing, a virtual ray is projected onto the three-dimensional object 100 as the observation object from a direction that a user specifies, and the three-dimensional object 100 is displayed as an image D (θ=0°). In this case, in the MIP processing, as a bright portion (maximum value) is displayed and objects positioned in front and at the back of the bright portion are not displayed, depth information is not represented.
Consequently, a parameter value, which is a virtual ray (camera) direction, is changed by 5° at a time from θ=−15° to θ=15°, for example, whereby the three-dimensional object 100 is displayed as from an image A to an image G. Then, the images provided by oscillating the virtual ray (camera) direction from side to side and changing the parameter value within a range from θ=−15° to θ=15° are displayed sequentially. Such image processing is called a tumbler display function in the medical diagnosis using volume rendering images.
Hitherto, in such image processing, data within a maximum amplitude (θ=±15°) are sequentially calculated and displayed, and the calculation results that are once displayed are cached. After that, animation display is performed at high speed using the cache.
FIG. 12 is a schematic representation showing timings of displaying newly generated images and cached images. The horizontal axis indicates a projection angle of the virtual ray, and the vertical axis indicates time when the image is displayed. As shown in FIG. 12, in the related art, image data A, B, C, D, E, F, and G within the maximum amplitude (θ=±15°) are sequentially calculated and displayed. Thus, it takes a long time to calculate all the images. On the other hand, as the calculated image data A, B, C, D, E, F, and G are stored in a cache, after that, animation display is performed at high speed using the cache.
However, in the image processing method in the related art described above, it is required to calculate images at all angles within the maximum amplitude that is specified by the user. Therefore, a problem arises that time Δt1 taken to perform the first animation display is long.
For example, in a case where the medical diagnosis using images from the CT apparatus is conducted, it may take several ten seconds to calculate a plurality of images by the tumbler function with the angle that the user specifies as the center. Particularly, when the specified angle is changed or a scale factor of the image is changed, the user needs to wait for a long time.